Chongqing might be huge—but it's not the biggest city in China, much less the world. (Wikimedia Commons)

In 1997, the Chinese Communist Party designated the southwestern city of Chongqing as China's fourth "municipality," carving an area about the size of South Carolina out of Sichuan province. The purpose of the change was to transfer administrative control of the city from the Sichuan provincial government to the capital, enabling Beijing to better oversee the development of the nearby Three Gorges Dam project. Overnight, the Chinese government had created a conurbation of 28 million residents, leading Time magazine (among others) to label Chongqing "the world's largest city."

There was just one problem: Chongqing "municipality" has a lot of land that isn't, well, municipal: In addition to the city itself, Chongqing now includes outlying suburbs, satellite cities, and ample countryside. If we were to consider just the city's urban core, then Chongqing's actual population is closer to 5 million—still huge, but not the biggest city in China, much less the world.

Andrew Stokols

What accounts for the confusion? The answer lies in China's peculiar administrative divisions. First, there are ordinary provinces (like Sichuan, Yunnan, and Shanxi) which, like American states, have a parallel government structure to the central government. Then, there are "ethnic autonomous regions" (like Tibet, Xinjiang, and Inner Mongolia) which function like provinces and aren't, by the way, remotely autonomous. And finally, there are the municipalities: Beijing, Tianjin, Shanghai, and Chongqing, large cities governed directly by the central government. (Imagine if, in addition to Washington, D.C., that New York City, Chicago, and Los Angeles were "districts" rather than cities within states). Then, finally, are the special administrative regions of Hong Kong and Macao, but that's a whole other story.

So when we say that a Chinese city has a certain population, it's worth remembering that, in the case of municipalities, the actual "city" part is much smaller.

Andrew Stokols

Then, there's then the question of the hukou, a "household registration" document that functions as an internal passport by tying an individual's eligibility for social services like education and health care to his or her city of origin. Over the past three decades, hundreds of millions of rural Chinese people migrated to the cities in search of work, and settled despite not being official residents. Until recently, China's census only counted residents with an urban hukou as part of the population, further skewing the data.

Andrew Stokols

The Chinese government has made urbanization a central part of its transition from an export and investment-based economy to one relying on consumption, which it views as a more sustainable growth model. But while Chinese cities are indeed growing—and fast—a closer look at population data shows just how complex a problem urbanization really is.